(1) Field of the Invention
The invention relates to a method of completing a tungsten contact in an integrated circuit device, and more particularly, to a method of completing a tungsten contact while maintaining underlayer planarity.
(2) Description of the Prior Art
Blanket etchback of tungsten is known. In his book, Chemical Vapor Deposition of Tungsten and Tungsten Silicides (by John E. J. Schmidt, Noyes Publications, Park Ridge, NJ, c. 1992, pp. 43-50), Schmidt describes the etchback of blanket tungsten either with or without a sacrificial layer. The problem of tungsten residue is not addressed.
Referring to FIG. 1, there is shown a portion of a partially completed integrated circuit in which there is a silicon substrate 10. Also shown are dielectric layers 12 and 16 and a polysilicon layer 14. Contact or via opening 17 is shown to a source/drain region 18. A shorter contact or via opening 19 contacts polysilicon layer 14. A layer of tungsten 20 is blanket deposited over all. A barrier or adhesion layer such as titanium nitride may be used under the tungsten layer, but is not illustrated.
FIG. 2 shows the same portion of the integrated circuit after tungsten etchback. There are four problem areas. In area 1 where the slope of the indentation is 40% or more, a residue of tungsten layer 20 remains after etching. In area 2, where the slope is 30% or more, some tungsten residue remains. In area 3, the contact is about 1 micrometer in height. There is a dimple in the tungsten plug. In area 4, where the contact is less than about 0.6 micrometers in height, the plug may have a dimple which goes all the way to the underlying layer. If the tungsten is etched enough to remove the residue in areas 1 and 2, the entire plug is area 4 will be etched away and damage may occur to the underlying layer 14. If tungsten residue exists after the tungsten etchback, leakage will occur from the residue into overlying layers. Overetching to remove the residue results in the loss of the tungsten plug in short via holes. Another drawback of the conventional process is that surface roughness of the tungsten is transferred to the underlayer silicon oxide layer when the tungsten is etched back. Because of this, there is a critical requirement for underlayer planarity necessitating a higher flow temperature for the insulator flow process.
U.S. Pat. No. 5,164,330 to Davis et al describes a method of etching a tungsten layer using nitrogen trifluorid (NF.sub.3) and argon gases to reduce residue buildup.